In conventional physiological signal amplifying circuitry, the action potential picked up by a plurality of electrodes attached to a body is routed, through a buffer amplifier, to the lead selector which produces a desired lead signals by combining input action potential, the output of the selector being supplied to the first stage of the differential amplifiers. In such amplifier circuitry, in order to increase the common mode rejection ratio, it is necessary to make fine adjustment of the resistors for removing the common mode signals within the differential amplifiers. In addition, when the electronic switches are employed in the lead selector, variety of internal resistances of the electronic switches causes variation of the common mode rejection ratio.